LETTER doi:10.1038/nature12872 Three keys to the radiation of angiosperms into freezing environments Amy E. Zanne1,2, David C. Tank3,4, William K. Cornwell5,6, Jonathan M. Eastman3,4, Stephen A. Smith7, Richard G. FitzJohn8,9, Daniel J. McGlinn10, Brian C. O’Meara11, Angela T. Moles6, Peter B. Reich12,13, Dana L. Royer14, Douglas E. Soltis15,16,17, Peter F. Stevens18, Mark Westoby9, Ian J. Wright9, Lonnie Aarssen19, Robert I. Bertin20, Andre Calaminus15, Rafae¨l Govaerts21, Frank Hemmings6, Michelle R. Leishman9, Jacek Oleksyn12,22, Pamela S. Soltis16,17, Nathan G. Swenson23, Laura Warman6,24 & Jeremy M. Beaulieu25 Early flowering plants are thought to have been woody species to greater heights: as path lengths increase so too does resistance5. restricted to warm habitats1–3. This lineage has since radiated into Among extant strategies, the most efficient method of water delivery almost every climate, with manifold growth forms4. As angiosperms is through large-diameter water-conducting conduits (that is, vessels spread and climate changed, they evolved mechanisms to cope with and tracheids) within xylem5. episodic freezing. To explore the evolution of traits underpinning Early in angiosperm evolution they probably evolved larger conduits the ability to persist in freezing conditions, we assembled a large for water transport, especially compared with their gymnosperm cousins14. species-level database of growth habit (woody or herbaceous; 49,064 Although efficient in delivering water, these larger cells would have species), as well as leaf phenology (evergreen or deciduous), diameter impeded angiosperm colonization of regions characterized by episodic of hydraulic conduits (that is, xylem vessels and tracheids) and climate freezing14,15, as the propensity for freezing-induced embolisms (air bub- occupancies (exposure to freezing). To model the evolution of spe- bles produced during freeze/thaw events that block hydraulic pathways) cies’ traits and climate occupancies, we combined these data with an increases as conduit diameter increases5. Three evolutionary solutions unparalleled dated molecular phylogeny (32,223 species) for land liidae plants. Here we show that woody clades successfully movedintofreezing- Magno prone environments by either possessing transport networks of small 5 M o safe conduits and/or shutting down hydraulic function by dropping n o c leaves during freezing. Herbaceous species largely avoided freezing o ty le periods by senescing cheaply constructed aboveground tissue. Growth d e o 6 a n id e habit has long been considered labile , but we find that growth habit s a o e r r was less labile than climate occupancy. Additionally, freezing envir- e p u onments were largely filled by lineages that had already become herbs S or, when remaining woody, already had small conduits (that is, the trait evolvedbefore the climate occupancy). By contrast, most decidu- ous woody lineages had an evolutionary shift to seasonally shedding their leaves only after exposure to freezing (that is, the climate occu- pancy evolved before the trait). For angiosperms to inhabit novel cold environments they had to gain new structural and functional trait solutions; our results suggest that many of these solutions were probably acquired before their foray into the cold. Flowering plants (angiosperms) today grow in a vast range of envir- S u p e r onmental conditions, with this breadth probably related to their diverse a s t e 7 r i d morphology and physiology . However, early angiosperms are gen- a erally thought to have been woody and restricted to warm understory e habitats1–3. Debate continues about these assertions, in part because of Figure 1 | Time-calibrated maximum-likelihood estimate of the molecular the paucity of fossils and uncertainty in reconstructing habits for these 8–11 phylogeny for 31,749 species of seed plants. The four major angiosperm first representatives . Nevertheless, greater mechanical strength of lineages discussed in the text are highlighted: Monocotyledoneae (green), woody tissue would have made extended lifespans possible at a height Magnoliidae (blue), Superrosidae (brown) and Superasteridae (yellow). necessary to compete for light12,13. A major challenge resulting from Non-seed plant outgroups (that is, bryophytes, lycophytes and monilophytes) increased stature is that hydraulic systems must deliver water at tension were removed for the purposes of visualization. 1Department of Biological Sciences, George Washington University, Washington DC 20052, USA. 2Center for Conservation and Sustainable Development, Missouri Botanical Garden, St Louis, Missouri 63121, USA. 3Department of Biological Sciences, University of Idaho, Moscow, Idaho 83844, USA. 4Institute for Bioinformatics and Evolutionary Studies, University of Idaho, Moscow, Idaho 83844, USA. 5Department of Ecological Sciences, Systems Ecology, de Boelelaan 1085, 1081 HV Amsterdam, the Netherlands. 6Evolution & Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales 2052, Australia. 7Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan 48109, USA. 8Department of Zoology and Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia V6T1Z4, Canada. 9Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia. 10Department of Biology and the Ecology Center, Utah State University, Logan, Utah 84322, USA. 11Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996, USA. 12Department of Forest Resources, University of Minnesota, St Paul, Minnesota 55108, USA. 13Hawkesbury Institute for the Environment, University of Western Sydney, Penrith, New South Wales 2751, Australia. 14Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA. 15Department of Biology, University of Florida, Gainesville, Florida 32611, USA. 16Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611, USA. 17Genetics Institute, University of Florida, Gainesville, Florida 32611, USA. 18Department of Biology, University of Missouri—St Louis, St Louis, Missouri 63121, USA. 19Department of Biology, Queen’s University, Kingston, Ontario K7L 3N6, Canada. 20Department of Biology, College of the Holy Cross, Worcester, Massachusetts 01610, USA. 21Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, United Kingdom. 22Polish Academy of Sciences, Institute of Dendrology, 62-035 Kornik, Poland. 23Department of Plant Biology and Ecology, Evolutionary Biology and Behavior, Program, Michigan State University, East Lansing, Michigan 48824, USA. 24Institute of Pacific Islands Forestry, USDA Forest Service, Hilo, Hawaii 96720, USA. 25National Institute for Mathematical & Biological Synthesis, University of Tennessee, Knoxville, Tennessee 37996, USA. 6 FEBRUARY 2014 | VOL 506 | NATURE | 89 ©2014 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER seemingly arose to address the challenges of freezing: (1) woody species .0 uC across a species’ range; and ‘freezing exposed’, encountering withstood freezing temperatures without serious loss of hydraulic func- temperatures #0uC somewhere across a species’ range. This dichotomy tion by building safe water-transport networks consisting of small-diameter assumes that climate tracking through environmental changes is more conduits; (2) woody species shut down hydraulic function by becom- common than the evolution of climate occupancy; this is more likely to ing deciduous, dropping leaves during freezing periods; and (3) herb- be true if freezing exposure has a physiological cost in regions without aceous species largely avoided freezing by senescing cheaply constructed freezing21. Species were further distinguished by leaf phenology (deciduous aboveground tissue and overwintering, probably as seeds or underground or evergreen); conduit diameter (large $0.044 mm, or small ,0.044 mm; storage organs. However, the order in which angiosperms are likely to as 0.044 mm diameter is the diameter above which freezing-induced have acquired these solutions relative to exposure to and persistence in embolisms are believed to become frequent at modest tensions22); and the cold16 remains unclear. growth form (woody or herbaceous, with woody species defined as Proportions of herbaceous species, deciduous species and those with those maintaining a prominent aboveground stem that is persistent small water-conducting conduits increase towards the poles1,4,17,18, and over time and with changing environmental conditions; see Extended an earlier limited survey of angiosperm families indicated that herba- Data Fig. 1 for examples of angiosperms with woody growth habits as ceousness and ability to cope with freezing evolved in parallel19.However, we define them, and Extended Data Table 1 for a breakdown of growth exactly how global-scale ecological patterns are linked to functional evolu- habit by order within angiosperms). tion of angiosperms is uncertain. We dissect the contributions of different Among woody species we asked whether evolutionary transitions evolutionary solutions allowing angiosperms to cope with periodic freez- between climate occupancy states were significantly associated with shifts ing and assess likely pathways by which clades acquired these traits (that is, in leaf phenology and/or conduit diameter. Among all angiosperms we timing of evolution